The interindividual variability in breast tissue on mammographic images, as defined by several measures of mammographic breast density, has been shown to be a major risk factor for breast cancer. However, there are several limitations in the studies performed to date; they have involved older mammogram images from multiple institutions, using mostly subjective measures on one view of the breast. Also, none of these studies have incorporated biological samples into their analyses and very few have examined other features that may be more predictive of breast cancer risk. Our GOAL is to conduct a large-scale, prospective study at one institution with new mammography to examine the causal association of breast density with breast cancer and to identify new markers of breast cancer risk. The Mayo Mammography Clinic performs approximately 35,000 screening mammograms per year; 25,000 of these are from Minnesota, Wisconsin and Iowa. Over a three-year period, we will enroll 20,700 cancer-free women from this tri-state region into our mammography cohort study. We will collect complete risk factor information from a baseline questionnaire, modern mammogram films and biological samples and will follow participants/members for breast cancer incidence and mortality. We will use a semi-automated algorithm to estimate percent breast density, rate of change of percent density over time, dense area and regional density on members of our cohort. We propose to examine aspects of breast density with incidence of breast cancer; specifically, we will examine percent breast density, novel features of the mammogram including regional density, total dense area and longitudinal rate of change of breast density. Additionally, we will ascertain machine variability and settings (kVp, mAs, compression, thickness) from mammogram films and incorporate into the above analyses. As secondary aims, we also propose to investigate the heterogeneity of change in breast density among women who initiate hormone replacement therapy (HRT). The literature shows that approximately 30 percent of women experience increases in breast density upon HRT initiation. We propose to examine whether there is an association between this change in breast density and risk of breast cancer and also whether there is an association between this change and four functional polymorphisms in the genes SULT1A1, SULT1E1 and CYP3A5. The findings from these proposed studies have obvious translational implications: high-risk groups can be identified for intervention, more aggressive surveillance and perhaps chemoprevention. [unreadable] [unreadable]